This invention relates to water chlorination systems and in particular to bypass chlorinators for treating water flowing through a pipe. This invention was developed to provide a means for chlorinating fresh water aboard ships. One of the most common of the prior art systems used for chlorinating water aboard ships is a controllable displacement pump which injects a concentrated chlorine solution from a storage tank into a fresh water pipe to purify the water flowing through the pipe. The concentrated chlorine solution is prepared by dissolving granular calcium hypochlorite powder in the fifteen to thirty gallon chlorine solution storage tank. The output of the injector pump is adjusted to provide the desired treatment rate. The injector pumps are driven by either an electric motor or a water meter drive. With this type of chlorination system, the injection rate of the chlorine solution into the fresh water is difficult to control. The system will not work if the pump fails or if the electric motors or water meters driving the pump fail. The calcium hypochlorite powder tends to be dangerous to handle and store and mixing the powder with water to form the chlorine solution is very time consuming. Other prior art devices chlorinate water flowing through a pipe by diverting a portion of the water from the pipe, dissolving tablets of a chlorine compound such as sodium hypochlorite in this diverted water, and mixing the resulting concentrated chlorine solution with the undiverted water flowing through the pipe. This type of system does not require special pumps or outside power sources because the force of the water flowing through the pipe will make it work. However, with this bypass type of chlorinator system it is difficult to control the rate in which the concentrated chlorine solution is added to the water flowing through the pipe. The concentration level of chlorine in water treated by these prior art bypass devices tends to change both as a function of the water pressure in the pipe and also as a function of the water flow rate through the pipe. In an ideal bypass water chlorinator, the concentration of the dissolved chlorine compound in the treated water produced by the chlorinator would be independent of both the flow rate and the water pressure within the pipe. It is also desirable to build such chlorinators so that it will be safe, fast and convenient to refill them with sodium hypochlorite tablets.